Nickel-based electrodes are commonly used in rechargeable electrochemical cells. For example, nickel hydroxide particles, Ni(OH).sub.2, usually constitute the positive electrode in both nickel-cadmium and nickel-metal hydride cells. Ni(OH).sub.2 is the material of choice for positive electrodes in both types of cells because it can offer high energy density as well as good rate capability, desirable properties in today's battery market. High energy density is obtained through use of pasted electrodes in which a paste comprising high density spherical Ni(OH).sub.2 particles is applied to a foam substrate; high rate performance is typically obtained by using sintered Ni(OH).sub.2 electrodes.
The crystal structure of Ni(OH).sub.2 is characterized by a hexagonal unit cell with a layered structure comprising one nickel, two oxygen, and two hydrogen atoms per cell, as illustrated in FIG. 1. This ".beta.-Ni(OH).sub.2 " layered structure can also be described as a system of lamellar plates comprising an arrangement of nickel and oxygen atoms. When the typical .beta.-Ni(OH).sub.2 electrode is charged, the positive electrode is oxidized and the Ni(II) of .beta.-Ni(OH).sub.2 releases one electron to become Ni(III) and form beta nickel oxyhydroxide, .beta.-NiOOH. In .beta.-NiOOH, the lamellar plates of the crystal become slightly displaced away from each other, changing the volume of the unit cell. Upon discharge, the positive electrode is reduced, the Ni(III) of .beta.-NiOOH accepting one electron to convert back to Ni(II) and form .beta.-Ni(OH).sub.2, whereby the plates return to their initial positions.
Because it is desirable to increase cell capacity, a number of approaches have been tried to facilitate or enhance electron-transfer in the positive electrode. One such method, described in U.S. Pat. No. 5,569,562 to Glemser et al., involves the incorporation of Mn(II) into the positive material in order to facilitate complete discharge thereof by increasing the efficiency of the one-electron Ni(III).revreaction.Ni(III) conversion. U.S. Pat. No. 5,569,563 to Ovshinsky et al. similarly employs carbon in the positive material to facilitate electron transfer.
Another approach has been to increase the degree of electron exchange to above one electron per nickel atom by forming higher-oxidation state materials within the positive electrode. Such materials include .gamma.-NiOOH, a material comprising both Ni(III) and Ni(IV), e.g., in the form of species including nickelate, (NiO.sub.2).sub.3.sup.-, in which the nickel atoms have fractional formal valences such as 32/3. Another such positive electrode material comprises regions of stable .alpha.-Ni(OH).sub.2 which convert to .gamma.-NiOOH upon charging. In contrast to the single-electron Ni(III).revreaction.Ni(II) conversion of .beta.-NiOOH, the presence of .gamma.-NiOOH within the charged electrode can increase the electron exchange to above 1 electron per nickel atom, through the participation of Ni(IV). Thus, in charged nickel electrodes containing .gamma.-NiOOH, both Ni(III) and Ni(IV) are converted to Ni(II) upon discharge, thereby increasing the degree of electron exchange. For example, in this scenario, were the nickel content of the positive material to comprise half Ni(III) and half Ni(IV), complete discharge would theoretically increase the degree of electron exchange to an average of 1.5 electrons per nickel atom. If such an effect were stable, this would increase the electrical capacity of the electrochemical cell.
However, in .gamma.-NiOOH, the lamellar plates of the crystal become significantly displaced away from each other, greatly expanding the crystal unit volume. FIG. 2 illustrates the differences in crystal structure among .beta.-Ni(OH).sub.2, .beta.-NiOOH, and .gamma.-NiOOH. This expansion cracks the crystals into smaller particles and greatly increases the total internal surface area and the porosity of the positive electrode. The greater surface area results in the creation of dry areas in the separator. These dry areas are susceptible to oxidation which leads to the generation of gases within the cell. These effects cause cells to fail or radically shorten their life.
In response to these problems, manufacturers of rechargeable batteries regularly use Ni(OH).sub.2 which has been produced by co-precipitation with anti-.gamma. additives, such as cadmium and/or zinc metals or compounds. The additives interfere with .gamma.-NiOOH formation, apparently by occupying the spaces between lamellar plates of the .beta. crystal structures and interacting with the plates to prevent their extensive displacement to the .gamma. structure. Without the anti-.gamma. additives, residual charge remaining in the positive after incomplete discharge would accumulate, ultimately converting the .beta.-NiOOH to .gamma.-NiOOH as depicted in FIG. 2.
The following series of reactions illustrates the charge-discharge cycle relationships between the various forms of Ni(OH).sub.2 and NiOOH. ##STR1## As shown, the most stable structure is the uncharged form, .beta.-Ni(OH).sub.2. With normal charging, .beta.-Ni(OH).sub.2, an Ni(II) form, converts to .beta.-NiOOH, an Ni(III) form. In the traditional nickel electrode, this form is reduced back to .beta.-Ni(OH).sub.2 upon discharge. If instead, .beta.-NiOOH is further charged, i.e. "overcharged," it converts to .gamma.-NiOOH, which contains Ni(IV). When .gamma.-NiOOH is discharged, it mainly reverts directly to .beta.-Ni(OH).sub.2, although a small percent may convert first to the unstable intermediate, .alpha.-Ni(OH).sub.2.
The previously suggested methods of solving the above-noted problems with the use of .gamma.-NiOOH, or .alpha.-Ni(OH).sub.2, involve incorporating relatively high concentrations of various metals, metal hydrides, oxides, and alloys in Ni(OH).sub.2. For example U.S. Pat. Nos. 5,384,822 and 5,567,549 to Ovshinsky et al. describe the incorporation of compositional modifiers to create stable regions of .gamma.-NiOOH and/or .alpha.-Ni(OH).sub.2 within the positive. However, such methods require extra processing steps as well as the use of additional metals such as cesium and cobalt, increasing the cost needed both to make positives containing regions of these higher oxidation state phases, and to manufacture cells therefrom. Moreover, these stable-region methods fail to maximize the formation of .gamma.-NiOOH and present an inherent limit on the possible improvement in cell electrical capacity.